This invention generally relates to an isotopically altered optical fiber, and is specifically concerned with a silica fiber enriched with the heavier isotopes of oxygen and/or silicon in order to reduce transmission losses and to increase bandwidth.
Optical loss is a limiting factor in the design and construction of optical networks and links, which typically include hundreds of kilometers of silica-based optical fiber. Consequently, a reduction in the loss of the fiber by even hundredths of decibels per kilometer would have a significant impact on the performance of such networks. Optical losses in silica fibers are predominantly caused by two factors, including (1) Rayleigh scattering, which falls off as a function of 1/xcex4 and which dominates for shorter wavelengths, and (2) infrared absorption by the silica, which dominates for longer wavelengths. As is well known in the art, the product of these two forms of optical loss falls to a minimum at a wavelength of approximately 1560 nm. Consequently, most optical signals are transmitted at a bandwidth centered around 1560 nm in order to minimize transmission losses. At a bandwidth between 1510-1610 nm losses typically vary between a minimum of 0.189 dB/km and a maximum of 0.200 dB/km. As small as this may seem, such a loss rate still translates into a 50 percent loss of signal over a distance of 15 km, which is quite significant when one considers that some networks (such as the one traversing the Atlantic Ocean) are over 6,500 km long.
While there have been various attempts in the past to develop an optical fiber with lower transmissivity losses, the high costs associated with the manufacture of such fibers has prevented them from enjoying widespread use. So called silica core fibers, e.g., those having an undoped silica core and a fluorine doped cladding, are known which have reduced losses on the order of 0.151 db/km at 1550 nm are known. However, it is well known that silica core fibers are much more difficult to manufacture than fibers having cores which include index of refraction-altering dopants. Other types of fibers with potential for low loss are known which employ a non-silica chemistry. However, the materials used in such fibers are far more expensive than silica, and cannot be drawn and worked on a commercial scale without the development of completely different kinds of manufacturing equipment than is presently in use.
The invention relates to an optical waveguide comprising multiple isotopes of a same chemical element in relative proportions sufficiently changed from a naturally occurring proportion of the isotopes such that the optical losses are reduced in the vicinity of the standard 1510-1610 nm bandwidth, the bandwidth is substantially increased, and the Raman spectrum is broadened. The waveguide preferably is an optical fiber.
In one embodiment, the waveguide includes a light conducting core region comprised of silica glass, and at least a portion of the oxygen in the silica is comprised of multiple isotopes of oxygen in relative proportions which are changed from a naturally occurring proportion of oxygen. In particular, oxygen-18 makes up greater than 20 mole percent, more preferably greater than 50 mole percent, even more preferably greater than 70 mole percent, and most preferably greater than 80 mole percent of the total amount of oxygen in the core region. At the same time, in any of the embodiments disclosed herein wherein the amount of oxygen-18 is increased in the waveguide over that which is naturally occurring, the amount of oxygen-17 is likewise increased, preferably at least 5 percent, more preferably at least 10 percent, and most preferably at least 15 percent of the amount of oxygen-18 employed in the fiber. Consequently, for example, when the oxygen-18 is greater than 50 mole percent of the total oxygen present, it is preferable that the oxygen-17 be greater than about 2.5 mole percent, more preferably at least 5 mole percent, and more preferably greater than about 7.5 mole percent, of the total molar oxygen content.
As the losses due to Rayleigh scattering remain about the same for wavelengths in the range of about 1500 to 1800, nm while the losses due to infrared absorption in this range are reduced, the net effect is that a new minimum transmission loss of approximately 0.145 dB/km occurs near wavelengths of about 1670 nm. Because the loss curve with respect to wavelength is flatter in the vicinity of the new minimum, bandwidth over a variation of 0.010 dB/km (i.e., in a loss range of between 0.145 and 0.155 dB/km) is increased 17 percent over the 100 nm bandwidth of conventional germanium-doped fiber. If a maximum loss rate of about 0.19 dB/km can be tolerated (which is the minimum loss of the lowest loss germanium-doped fiber presently commercially available), bandwidth can be increased 100 percent. Finally, if a maximum loss rate of 0.200 dB/km can be tolerated (which is the loss rate of conventional fibers) bandwidth can be increased 275 percent.
While the resulting reduction of transmission losses between 0.034 and 0.044 dB/km would not appear to be large, such a reduction would translate into substantial savings, particularly in long distance transmission networks. For example, when conventional fiber is used to form a network traversing the Atlantic Ocean, the optical losses which occur necessitate signal regeneration stations for every 125 km of fiber. With the reduction in transmission losses of only between 0.034 and 0.044 dB/km, such regeneration stations are required only every 156 km. This would result in a net reduction of 11 regeneration stations. As such ocean based stations cost approximately one million dollars a piece, the net savings in a transatlantic transmission line amounts to over $11 million.
Another embodiment of the invention relates to an improved Raman gain fiber, the full width half maximum of the Raman gain spectrum in such fibers may be increased at least 5 percent over fibers made using naturally occurring oxygen. In this particular embodiment of the invention, the enrichment of either or both of the oxygen or silicon in the core region of the fiber with one of the heavier isotopes of either oxygen or silicon does not have to amount to a complete substitution, as it may be in other embodiments. Rather, if oxygen-18 is used as the enriching isotope, only about half of the oxygen in the core region need be oxygen-18, the balance being oxygen-16. The reason for such an incomplete substitution is that a complete substitution results in a complete shifting of the Raman gain spectrum, whereas a roughly 50 percent substitution of oxygen-18 for the oxygen-16 has the more advantageous effect of broadening the Raman gain spectrum by the superposition of an ordinary Raman gain spectrum with the shifted Raman gain spectrum which would be obtained if only oxygen-18 were used in the silica forming the core region. For example, in one embodiment, the core of the Raman gain fiber is comprised of layers of glass which alternate between layers which are enriched with oxygen-18, as described above, and layers of glass which are not enriched beyond that which is present in naturally occurring oxygen. Consequently, for embodiments involving Raman gain fiber, the level of substitution of oxygen-18 for oxygen-16 is preferably between about 30-70 mole percent, more preferably between about 40-60 mole percent, and most preferably between about 45-55 mole percent.